Laboratory Study of the Distribution of Grain Sizes in Cross-Bedded Deposits

The author gratefully acknowledges the support of the Water Resources Division of the U. S. Geological Survey, and he wishes to express his appreciation to Survey personnel for their continued advice and assistance. He also wishes to extend thanks to Drs. H. A. Thomas, Jr, R. Siever and B. Kummel for their interest and constructive criticism of the work; the help and inspiration afforded the author by the late Dr. J. P. Miller of Harvard University is gratefully acknowledged.

Abstract

A preliminary study was made of the movement and dispersion of particles in the flow transition over the foreset front of a tabular unit of cross–bedding (a micro–delta). The approximate path lines of particles settling out on the bottomset were derived by making simplifying assumptions concerning the flow pattern over the foreset front. Specifically, the path line for any given particle was derived by plotting the vectorial resultant of the longitudinal velocity of stream flow in the transition region and the settling velocity of the particle.

The velocity, depth, and suspended sediment concentration were determined for the flow upstream of the foreset slope and the path lines of the various size grades of particles were plotted for several basins of differing depths. The theoretical grain size compositions for ‘synthetic’ samples at selected positions along the bottom– set were calculated by integrating the path line data for the various size grades. The assumption was made that the relative proportions of particles of differing size grades settling out on the bottomset in unit time would determine the average composition at the selected sample position. There was a reasonable measure of agreement between predicted and actual grain size compositions, suggesting that the path line hypothesis affords a rational approach to the problem of downstream sorting.

The principle involved in the path line method can be used to a limited extent in the reconstruction of paleo– flow regimes. This is based on the premise that the dispersion of grain sizes in a bottomset affords an index to the depth of the stream. The latter can be estimated from an analysis of ‘inverse’ path lines.

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This volume contains papers presented as part of a symposium held in Toronto on May 18, 1964. These papers are mainly designed to assist the geologist, who is interested in the hydrodynamics of formation of sedimentary structures but who has little or no training in hydraulics, to become familiar with the extensive body of research which has been undertaken by hydraulic engineers interested in sediment problems.